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Saturday, 21 May 2011

Inductor selection for Switch Mode Supplies

Recently at work I have had to design a few switch mode power supply circuits. I haven't had too much experience at designing these before and have so far got away with following recommended circuit topologies in the switcher datasheet. This has generally worked out O.K. but you can end up with an over-specified inductor which has many implications for the performance of the final circuit. Another problem with this approach is that blindly trusting the manufacturers datasheet examples is bound to lead to problems eventually. The design criteria for such circuits are rarely specified beyond output voltage/output current, there is no mention of ripple current/voltage, overshoot, power dissipation...

I set out to get a better understanding of how switch mode power supplies work, and understand the equations which underline their performance. This post is a collection of the resources I found during the learning process.

My starting point was SMPS Technology which has a short tutorial on the basics of switching regulators. This outlines the basic inductor equations and how to apply these to a buck regulator. It offers suggestions on choosing suitable inductor and capacitor values based on simple 'rules of thumb'. Helpfully, it explains the different approaches to controlling switch mode supplies and the effects of each method on stability and overshoot.

Having read this tutorial through a few times, I was keen to understand where these rules of thumb had come from and what the implications of following them were. After a trawl of switching regulator manufacturer's websites, I came across a National Semiconductors Application Note 1197. This is an expertly written document which explains how to select an appropriate inductor for your application. As well as showing how to choose a starting value for your inductor, it explains how to calculate the RMS current, Peak current, energy handling capability and saturation current required for you application. Furthermore, it shows how you can take the parameters from an Inductor datasheet and calculate the power losses due to core losses and copper losses and therefore work out the temperature rise.

If you want to be even more lazy you can try the National Semiconductor WEBENCH tool, which will not just select an inductor but will design the whole switch mode circuit for you, including sourcing all the components! I tried using this for the last switch mode circuit I designed to see how good it was. I was sceptical that it would do a good job. The circuit topology it suggested was pretty much the one from the datasheet with some important exceptions. It suggested an output capacitor with a large ESR which would have given poor voltage ripple. It violated the maximum resistance for the feedback resistors and it missed of the soft start capacitor. In conclusion, don't trust it to get everything right just yet!